Cheng Zhiliang, Gilmore Reid
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 364 Plantation Street, Worcester, Massachusetts 01605-2324, USA.
Nat Struct Mol Biol. 2006 Oct;13(10):930-6. doi: 10.1038/nsmb1146. Epub 2006 Sep 17.
Integral membrane proteins are cotranslationally inserted into the endoplasmic reticulum via the protein translocation channel, or translocon, which mediates the transport of lumenal domains, retention of cytosolic domains and integration of transmembrane spans into the phospholipid bilayer. Upon translocon binding, transmembrane spans interact with a lateral gate, which regulates access to membrane phospholipids, and a lumenal gate, which controls the translocation of soluble domains. We analyzed the in vivo kinetics of integration of model membrane proteins in Saccharomyces cerevisiae using ubiquitin translocation assay reporters. Our findings indicate that the conformational changes in the translocon that permit opening of the lumenal and lateral channel gates occur less rapidly than elongation of the nascent polypeptide. Transmembrane spans and lumenal domains are therefore exposed to the cytosol during integration of a polytopic membrane protein, which may pose a challenge to the fidelity of membrane protein integration.
整合膜蛋白通过蛋白质转运通道（或转运体）共翻译插入内质网，该通道介导腔结构域的转运、胞质结构域的保留以及跨膜片段整合到磷脂双分子层中。在转运体结合后，跨膜片段与一个侧向门相互作用，该侧向门调节对膜磷脂的 access，以及一个腔门，该腔门控制可溶性结构域的转运。我们使用泛素转运分析报告基因分析了酿酒酵母中模型膜蛋白整合的体内动力学。我们的研究结果表明，允许腔通道门和侧向通道门打开的转运体构象变化发生的速度比新生多肽的延伸速度慢。因此，在多聚体膜蛋白整合过程中，跨膜片段和腔结构域暴露于胞质溶胶中，这可能对膜蛋白整合的保真度构成挑战。
